Transgenic Approaches to Combat Fusarium Head Blight in Wheat and Barley

of the challenges to achieve successful transformation, prospects for transgene-mediated disease resistance, Fusarium head blight (FHB) and contamination with deoxynivacurrent research on candidate antifungal genes and prolenol (DON) produced by the primary pathogen Fusarium gramimoters, and strategies for optimal antifungal activity. nearum Schwabe [teleomorph Gibberella zeae (Schwein.)] have caused devastating losses to wheat (Triticum aestivum L.), durum (Triticum turgidum L. ssp. durum Desf.Husr.), and barley (Hordeum Transformation Methods vulgare L.) growers across the USA since the early 1990s. Evaluation The first reports of successful production of fertile of barley, wheat, and related germplasm yielded only a few accessions transgenic barley were published in 1994. These experiwith partial resistance. This resistance appears, in most cases, to be ments used particle bombardment (biolistics) to insert under polygenic control, making the development of resistant cultivars the genes into barley tissue (Ritala et al., 1994; Wan with suitable agronomic and quality traits a challenge. The insertion of individual antifungal and antitoxin genes via genetic transformaand Lemaux, 1994). Since then, a number of laboratories tion has the potential to aid in development of resistant wheat and have reported transformation of barley with selectable barley cultivars. Although wheat and barley transformation has been genes such as bar for resistance to the herbicide bialachieved in several laboratories, the development of a high throughput aphos[L-2-amino-4-((hydroxy)methyl)(phosphinoyl)wheat and barley transformation systems has been slowed by genotype butyryl-L-alanyl-L-alanine], nptII for resistance to the effects on plant regeneration, low transformation efficiencies, soantibiotics kanamycin and G418, and visual reporter maclonal variation, and problems with transgene inheritance and stagenes such as uidA for b-glucuronidase (GUS) expresbility of expression. Among the antifungal genes targeted to combat sion, using a variety of target tissues (reviewed by LemFHB are coding sequences for proteins that degrade fungal cell walls, aux et al., 1999). In the last few years, research has disorganize fungal membranes, bolster the host defense response sysmoved beyond developing methods using selectable and tems, and interfere with fungal protein synthesis, pathogenesis, and/ or accumulation of DON. Promoter sequences have been selected screenable genes to inserting genes of interest into barthat confer high levels of expression to the antifungal constructs, parley (Lemaux et al., 1999). ticularly in the spike tissues which are susceptible to FHB. As more Tingay et al. (1997) developed an Agrobacteriumantifungal genes are inserted into wheat and barley, field and greenmediated method for transforming immature barley house evaluation will show whether transgenes achieve their potential embryos, recovering plants from 54 independently in the fight against FHB. transformed cell lines. Two advantages of the Agrobacterium-mediated method compared with the particle bombardment method are insertion of fewer copies of F head blight has caused devastating losses the transgenes in stably transformed plants and the prefto wheat, durum, and barley growers and industry erential integration of the transgenes into transcriptionin the USA, particularly in the 1990s (McMullen et al., ally active chromosome regions (Lemaux et al., 1999). 1997). Researchers searching for resistance have found The first report of successful production of fertile a limited number of barley and wheat genotypes that transgenic wheat plants was by Vasil et al. (1992). They may provide partial resistance to the Fusarium species used the biolistic method to introduce DNA into 5to that cause FHB (Rudd et al., 2001). No sources of im7-mo-old embryogenic callus cultures and successfully munity to the disease have been identified (Gocho and regenerated plants containing, expressing and transmitHirai, 1987; McKendry et al., 1999; Prom et al., 1997). ting bar, the selectable marker gene encoding resistance The resistance that has been identified is under the to the phosphinothricin(L-PPT) based herbicide, control of multiple genes, creating a challenge for wheat BASTA (glufosinate). Within the next 2 yr, several inand barley breeders to develop resistant cultivars that dependent laboratories, beginning with Weeks et al. meet quality and agronomic standards. (1993), published variations of the method that is still Recent advances have made possible the insertion of most widely used today for wheat transformation individual genes into cereals including wheat and barley. (Becker et al., 1994; Nehra et al., 1994; Vasil et al., A variety of antifungal genes have been isolated, and 1993). This method targets embryogenic cells derived some of their products have been shown to inhibit Fufrom the scutellum of immature embryos of highly resarium growth in vitro and in planta. The development generable hexaploid wheat genotypes such as ‘Bobof transgenic wheat and barley expressing these genes white’, ‘Pavon’, ‘Fielder’, and ‘Florida’. In these espemay help in the fight against FHB. This paper reviews cially responsive varieties, scutellar cells readily form transformation methods used in wheat and barley, some embryogenic callus within a few days of culture in the presence of the auxin 2,4-dichlorophenoxy acetic acid L.S. Dahleen, USDA-Agricultural Research Service, Red River Val(2,4-D). Regeneration, even after bombardment and ley Agric. Res. Center, Fargo, ND 58105; P.A. Okubara and A.E. selection, is not limiting for transformation efficiency. Blechl, USDA-ARS, Western Regional Research Center, Albany, More recently, durum wheat has been transformed using CA 94710-1105. Received 28 March 2000. *Corresponding author ([email protected]). Abbreviations: FHB, Fusarium head blight; DON, deoxynivalenol. Published in Crop Sci. 41:628–637 (2001).